1. Field of Invention
The present invention relates to downhole oil well production logging. More particularly, the invention concerns a method and apparatus for measuring dielectric properties of downhole fluids in a cased oil well such as water hold-up, water cut, and water resistivity.
2. Description of Related Art
For many reasons, most petroleum companies today are concerned with maximizing the volume of oil produced by each of their oil wells. One reason for this is that many known hydrocarbon reserves have already been depleted, and exploring for new reserves is typically expensive. Moreover, substantial costs are usually required to drill new oil wells and install the necessary production equipment. As a result, it is desirable to operate a productive oil well in a manner that produces as much oil as possible, for as long as possible.
Many techniques are presently known for maximizing the volume of oil produced by an oil well. One of these, for example, is called "production logging." Production logging generally refers to the process of lowering a "tool string" into a producing oil well that has been cased and perforated. The tool string may include a number of well known devices for performing various functions, such as perforating the well casing, sealing perforations in the well casing, pumping petroleum from the well, measuring characteristics of fluids in the well, and the like.
Geophysicists are often interested in measuring characteristics of different fluids in the well, at different depths, to determine which depths of the well are producing oil, and the rate at which they are producing. Typically, perforations are made in the well casing at different depths to permit oil to flow into the well-bore from the surrounding strata. Although it is advantageous to create these perforations at depths corresponding to oil-bearing strata, these perforations are sometimes made at depths where a mixture of oil and water is located, or where water exists alone. In some cases, perforations are made at depths that initially produce a great deal of oil but, eventually produce more and more water and less and less oil, due to depletion of the reserves at that depth. If it is determined that a certain depth of the well is non-producing, or is producing mostly saline water ("brine"), some remedial work is performed upon that depth of the well. For example, the perforations in the casing at that depth may be plugged to stop production. Then, other more productive depths of the well may continue producing. Moreover, new production may be initiated by perforating the casing at other, untapped depths of the well. Therefore, an important function of production logging is to measure the ratio of water to oil at different depths inside the well casing.
"Water hold-up" (Y.sub..omega.) is defined as the ratio of water cross-sectional area (A.sub..omega.) to the total area (A) at a given depth in a fluid flow pipe, as shown in Equation 1 (below). ##EQU1## In Equation 1, h.sub..omega. is the cross-sectional area of water at a particular depth in the pipe. Accordingly, Y.sub.107 is actually the percentage of water in the pipe, at the specified depth, at a particular instant.
In contrast, "water cut" (C.sub..omega.) is defined as the amount of water produced by an oil well over a given period of time, expressed as a percentage of the total amount of fluid produced in that time period (Equation 2, below). ##EQU2## In Equation 2, Q.sub..omega. and Q are the volumetric flow rates of water and the total fluid, respectively, inside a fluid flow pipe. Water hold-up, then, is an instantaneous value, while water cut is a measurement over time.
If the average velocities of water (V.sub..omega.) and oil (V.sub.o) at a given location within a flow pipe are known, the relationship between water cut (C.sub..omega.) and water hold-up (Y.sub..omega.) can be written as shown in Equation 3 (below). ##EQU3## Water hold-up and water cut are only equivalent when all fluids are flowing at the same rate. For example, in a typical oil well, water hold-up is usually greater than water cut, due to the contrasting flow rates of water and oil or gas in the well.
A number of different instruments are presently used to measure water hold-up inside a well casing. First, there is the "capacitance probe." A capacitance probe is basically a capacitor that utilizes parallel plates to evaluate the content of fluid inside a well casing by measuring the fluid's capacitance as it flows between the plates. When capacitance probes measure the capacitive impedance of an oil-water mixture, it is assumed that oil is the continuous medium and entrained droplets of water are essentially conductive inclusions.
Capacitance probes have been helpful in a number of situations. For example, when the fluid mixture comprises globules of water suspended in oil (an "oil external" mixture), the mixture is effectively an insulator, and the capacitance probe may provide useful information.
However, in certain other applications, the usefulness of the capacitance probe is limited. For example, the accuracy of the capacitance probe may be adversely impacted due to varying levels of the salinity of the water, or due to other changes in the water's conductivity. Moreover, when the fluid mixture comprises globules of oil suspended in water (a "water external" mixture), the capacitance probe is inoperative. This drawback is especially limiting, since the primary application of capacitance probes is with failing oil wells that are producing more and more water, and less and less oil.
In practice, it is difficult to predict the oil-water ratio at which a "water external" mixture occurs because so many variables are involved; for example, flow-rate, temperature, water salinity, pressure, oil density, and other variables must be considered. These variables are typically unpredictable in downhole environments. Some arrangements have used "sample chambers" to help recognize the presence of a water external mixture. With this arrangement, flowing fluids are directed into a partially enclosed volume within the logging tool where the fluids are coupled to measurement electrodes. Although this approach may be useful in some applications, it may be difficult to ensure that a representative sample will be obtained, due to the tendency of oil-water mixtures to separate in low-turbulence environments. Therefore, even with sample chambers, capacitance probes are not as accurate as some might desire.
Another approach to measure water hold-up inside a well casing utilizes a "gamma ray densitometer." Basically, a gamma ray densitometer emits gamma rays, which are subsequently measured to quantify the density of fluids in a testing chamber. Theoretically, a densitometer can be used to determine water hold-up by measuring the density of the oil-water mixture inside the well casing. This is possible since the density of water is known, the density of oil is known, and the volume of the fluid in the testing chamber is known. Although the densitometer is useful in certain applications, some may encounter limitations under specific circumstances. For example, although the density of pure water is 1 gram/cc, the density of water inside an oil well casing may be higher due to the water's salinity. Moreover, the density of oil in the casing may vary between 0.7 and 0.9 grams/cc, depending upon the composition of the hydrocarbon materials in the well. For these reasons, then, the information provided by a densitometer may not be completely accurate. "Low energy radioactive meters", another device for measuring water hold-up, also suffer from some of the same problems as densitometers since they also measure density.
Another known technique for measuring water hold-up is the "gradiomanometer." Gradiomanometers measure pressure difference over fixed lengths of fluid. Techniques utilizing gradiomanometers suffer from some of the same problems as densitometers, since gradiomanometers also measure density. A gradiomanometer is typically aligned longitudinally with the well, and functions to determine fluid density by measuring the weight of a vertical column of fluid contained in a compartment of known weight and volume. A sensitive scale located beneath the compartment makes the weight measurement. Gradiomanometers are therefore inaccurate or inoperative when used with slant wells or horizontal sections of wells, since the fluid within the gradiomanometer is not completely vertical, and hence cannot be measured accurately by the gradiomanometer's scale.